Histoire plio-pléistocène des écosystèmes végétaux de Méditerranée sud-occidentale : apport de l’analyse pollinique de deux sondages en mer d’Alboran

The pollen analysis of sediments of two boreholes located in the northeastern Morocco (Nador 1) and in the southeastern Spain (Andalucia G1) has allowed shedding a new light on the vegetation and climate of the North African littoral plains, the Rif Massif and the Betic Range during the Pliocene. The vegetation around the Alboran Sea was open and xeric during the Zanclean, dominated by herbs including subdesertic elements as Calligonum, Lygeum, Nitraria and Neurada. This type of vegetation indicates a dry and hot climate. The southwestern Mediterranean steppes have therefore a climatic character; they existed before the presence and the heavy pressure of Man on the environment. From the Piacenzian, the development, at Andalucia as well as at Nador, of Artemisia and the appearance of some altitude trees such as Cedrus and Cathaya indicate a vegetation change linked to a climatic change. Modifications in the vegetation observed during the same period in northwestern Mediterranean seem to indicate that the vegetation changes observed at Andalucia and Nador are controlled by the appearance of the first arctic glacial-interglacial cycles. The cedar tree appears at Nador only at the end of the Piacenzian, at time of the first cooling, while it has been found punctually in Messinian and early Pliocene sediments of Habibas in Algeria and in Messinian sediments of the Bou Regreg section at Salé in Morocco. The regular presence of Cedar at Bou Regreg during the Messinian, allows supposing that it was present in the Middle Atlas Mountains and that its development was favoured by colder conditions.